1. Field
This invention relates generally to compositions and methods for the dissolution, disruption and/or removal of deposits from a heat transfer component.
2. Description of Related Art
It is typical for metal surfaces which are exposed to water or aqueous solutions over extended periods of time in closed heat transfer systems to develop scales and/or become covered by deposits. For example, in commercial nuclear power plants, on-line operation at high temperature can cause shell and tube heat exchangers, such as pressurized water reactor steam generators, to develop adherent scales and/or deposits via deposition or in-situ formation on the metal surfaces of its internal structural parts, such as secondary side surfaces of tubes, tube sheets, and tube support plates. In general, during nuclear power plant operation in a pressurized water reactor, high temperature, radioactive water flows from the reactor core through the inside of the heat exchanger tubes in the steam generator, transferring heat through the walls of the tubes and into the non-radioactive water surrounding the tubes. This causes the non-radioactive water to boil and create the steam that is used for power generation. During the boiling process, scale and other deposits can accumulate on the tube surfaces, in crevices between the tube support plates, on the tube walls and on horizontal surfaces, such as tube sheets and the surfaces of tube support plates. The accumulation of the scale and deposits on the internal structural parts of the steam generator over an extended period of time can have an adverse impact on the operational performance and integrity of the steam generators. For example, problems observed at operating nuclear power plants have included inefficient boiling heat transfer, obstruction of cooling water flow, and creation of flow occluded regions resulting in local aggressive corrosive environments impacting the structural integrity of the pressure boundary and structural materials.
Thus, various cleaning methods have been developed to remove these scales and deposits which build-up on the internal surfaces of heat exchangers used to generate steam, such as shell and tube heat exchangers, particularly, pressurized water reactor steam generators, by dissolving and disrupting deposits. Such cleaning methods can include chemical cleaning using a variety of chelating agents at elevated temperature, employing scale conditioning agents at elevated pH, and flushing with high pressure water. These processes typically result in a slow deposit removal rate under ambient temperature conditions. Further, the reaction rate is controlled by temperature shifts, pH shifts or an increase in the concentration of the chelating agent.
It is an object of the embodiments described herein to provide compositions and methods for at least partial dissolution, disruption and/or removal of deposits, such as scale and other deposits, from heat transfer components, particularly steam generators in pressurized water reactors. It is desirable for the compositions and methods to be effective in the absence of elevated temperature and/or effective in elevated pH conditions, for example, during routine plant refueling outages at an operating nuclear power plant.